70 GEOLOGY OF TDE HIGU PLATEAUS. 



district, though a few important ones are found in the northern part of the 

 field. The great western wall of the Awapa, the central and southern mass 

 of the Sevier Plateau, the southern Tushar and northern Markagunt, are 

 composed chiefly of such formations. The grand escarpments which wall 

 the imposing fronts of these plateaus are conglomerates, sometimes capped 

 with lava, sometimes intercalated, and more frequently without them. Near 

 the center of Grass Valley we have, on the east, bounding the western 

 verge of the Awapa, a wall of conglomerate which is more than 2,500 feet 

 thick ; and directly opposite, to the west, forming the eastern front of the 

 Sevier Plateau, is an exposure of very nearly equal magnitude, both stretch- 

 ing southward for 25 miles without interruption, save where erosion has 

 opened great gorges and ravines, though diminishing in thickness. From 

 a point a few miles southeast of Marysvale the western front of the Sevier 

 Plateau exhibits a wall of similar nature, extending south a distance of more 

 than 40 miles to the terminus of the plateau, with only two brief interrup- 

 tions. The southward expansion of the Sevier Plateau is made up chiefly 

 of such masses, and they reappear in the western flank of the Aquarius 

 beneath its monstrous lava cap. Their thickness will average here much 

 more than a thousand feet. In the northern part of the Markagunt they 

 appear to constitute the principal bulk of the area, though no deep expos- 

 ures are found and their thickness cannot even be conjectured. The south- 

 ern part of the Tushar rears a wall of similar nature, revealing nearly or 

 quite 2,000 feet of conglomerate, covering an area of at least 150 square 

 mihis, and probably very much more. The East Fork Canon is cut trans- 

 versely through the narrowest part of the Sevier Plateau, and exhibits on 

 either side a series of terraces rising J, 500 to 4,000 feet above the bed of the 

 stream. The lower 600 to 800 feet consist of "tufaceous" sandstones, and 

 above them are more than 2,500 feet of coarse conglomerate, with a few 

 massive sheets of intercalary lava. These clastic beds are everywhere seen 

 throughout the central and southern portions of the district and are built 

 upon a giant scale. 



Equally striking is the remarkable variety presented in their mechani- 

 cal texture and structure, whether we consider it in the hand specimen or 

 in the palisade and canon wall. We may consider them under two classes. 



